The ever increasing use of semiconductor light emitting devices (LEDs) has created a highly competitive market, wherein performance and reliability can significantly affect the success of a product.
FIGS. 1A-1D illustrate an example process for fabricating a semiconductor light emitting device. These figures illustrate a profile view of the device as it is being formed. One of skill in the art will recognize that a top view of the light emitting device could show a circular via 180, particularly if an etching process is used to form the via, or a trench/elongated via or an edge contact. For the purposes of this disclosure, and as detailed further below, a via is any opening that enables an electric conductor to connect an upper layer to a lower layer, through one or more intermediate layers.
In FIG. 1A, an N-type semiconductor layer 120 is grown upon a substrate 110, followed by an ‘active’ layer 130 and a P-type semiconductor layer 140. The active layer 130 emits light when a potential is applied between the N-type 120 and P-type 140 semiconductor layers.
In this example, the light that is emitted from the active layer is intended to exit the light emitting device through the N-type semiconductor layer 120; the substrate 110 may be transparent at least to wavelength the light emitted by the active layer, and/or it may be removed after the light emitting device is formed.
To increase the amount of light that exits the device, a reflective layer 150 may be formed above the P-type semiconductor layer 140. This reflective layer 150 redirects light toward the N-type semiconductor layer 120, reducing the likelihood of optical loss due to absorption within the light emitting device. In this example, the reflective layer 150 is patterned with an opening 155 that facilitates subsequent steps in the fabrication process, as detailed further below. Also in this example, the reflective layer forms the contact to the P-type semiconductor. The reflective layer 150 may include a silver (Ag) layer that is encapsulated in a barrier layer that minimizes the surface migration of the silver. The barrier layer may be, for example, a titanium tungsten (TiW), titanium tungsten nitride combination (TiWN), or titanium nitride (TiN). This reflective layer 150 is conductive, which facilitates current distribution across the P-type semiconductor layer 140.
In FIG. 1B, the P-type semiconductor layer 140 and active layer 130 are etched to create a via 180 that facilitates contact to the N-type semiconductor layer 120. Plasma-Ion isotropic etching 190 is commonly used to produce sloped via walls 185 within the via 180; these sloped walls 185 facilitate the application of a metal layer to contact the N-type semiconductor layer 120. The size of the via 180 at the opening 155 (FIG. 1A) at the reflective layer 150 is determined based on the width 125 of exposure of the N-type semiconductor layer 120 that is required to enable sufficient contact with the N-type layer 120 and sufficient separation from the active layer 130, as detailed further below.
Although the via 180 may have a single continuous wall around its perimeter, the plural term “via walls” is used herein to refer to the wall segments that appear in the cross-section view. As illustrated in FIG. 1B, the reflective layer 150 extends to the edge of the via 180, to maximize the reflective surface area.
In FIG. 1C, a dielectric layer 160 is formed to insulate the exposed N-type semiconductor layer 120 and the reflective layer 150 on the P-type semiconductor layer 140. This dielectric layer 160 is subsequently etched to enable select contact 165N, 165P to layers 120, 150 respectively, as illustrated in FIG. 1D. Grey shading is used to indicate insulation from the conductive layers, for ease of understanding.
After etching the dielectric layer 160 to enable contact 165N, 165P to the N-type semiconductor layer 120, and the conductive reflective layer 150 that contacts the P-type semiconductor layer 140, a metal layer 170 is applied. The metal layer 170 flows into the etched regions 165N and 165P to provide contact with the layers 120, 150. This metal layer 170 is patterned to create isolated metal segments 170N and 170P for connection to the N-type 120 and P-type 140 semiconductor layers, respectively.
The etching of the regions 165N and 165P is controlled to assure at least a minimum width 168N and 168P for adequate contact to the layers 120, 150. Additionally, a minimum separation 178 must be maintained between the contact of the N-type semiconductor layer 120 at the region 165N and the active region 130. The size 155 of the opening in the reflective layer 150 for forming the via 180 must also take into consideration the extent 188 of the active region 130 remaining after etching due to the sloped walls 185. Of particular note, the width 168N, the separation 178, and the extent 188 of the active region 130 remaining after etching are preferably as small as feasible, so that the size of the opening 155 may be as small as feasible, thereby maximizing the potential surface area for the reflective layer 150.
As can be seen, the minimum width/diameter of the via 180 at the opening 155 of the reflective layer 150 is equal to the width 168N of the contact plus twice the separation 178 plus twice the extent 188 of the active region 130 remaining after etching due to the slope wall 185.
The desire to maximize the coverage of the reflective layer 150, however, may introduce undesirable secondary effects. The extension of the reflective layer 150 to the edge of the via 180, for example, introduces an edge 161 in the dielectric layer 160 that extends vertically, which introduces a vertical drop 171 in the profile of the metal layer 170N. Gravity and other factors will cause a thinning of the metal layer 170N at the vertical drop 171 during fabrication, which may introduce gaps or cracks in the metal layer 170N, which may result in a premature failure of the device. A thicker coating of metal 170 may be applied, but this increases the cost of the device. The reflective coating 150 may be offset from the edge of the via 180, providing a more gradual descent, but this reduces the reflective surface area, resulting in more optical loss due to absorption.